Reddit has a thread on favorite first lines of books. I don't post there, but I was interested so I will get us started -

"The war ended, and the young men came home, and tried indignantly to fit themselves into old clothes and old habits which proved, on examination, to be both a little threadbare, and on trial to be both cripplingly small for bodies and minds mysteriously grown in absence."

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way - in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only.

A Tale of Two Cities, Charles Dickens

And one of my favorite lines ever is later in the book “It is a far, far better thing that I do, than I have ever done; it is a far, far better rest that I go to than I have ever known.” That line gave me a lot of comfort when it was time to put down my dog Buddy several years ago.

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way - in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only.

An amazing first line and a great book. I remember telling my High School English teacher that the last quarter of the book was so gripping that it almost made up for the very boring first three quarters.

As an aside, the Cheers episode where Frasier reads "A Tale of Two Cities" to the bar is hysterical. They quickly become board and he had to spice it up to keep their attention.

"It is a far, far better thing that I do, than I have ever done. It is a far, far better butt-kicking I give, than I have ever gave."

This isn't necessarily my favorite, but it's always stuck in my memory the way no other line has:

"It was a bright cold day in April, and the clocks were striking thirteen."

That's 1984, and I'm a bit embarrassed that part of the reason it hit me hard is because I didn't understand it properly. I didn't make the connection that Orwell was using a 24-hour time convention. I thought that Orwell was describing a society where they had decided to literally change the way time was measured and that there were now 13 hours in the first part of the day, like the French Republican calendar switching to a ten-day week. So I had that feeling of total disorientation and separation from Smith's world from literally the first sentence.

Years and years ago I brought my young children to a non denomination church. The visiting speaker that day discussed the first sentence of the Bible. He shortened the sentence to the first 4 words of the Bible ~ “In the beginning God”.